Vitamin C is found in foods primarily as ascorbic acid (80-90%), but dehydroascorbic acid (10-20%) is also present. The bioavailability of vitamin C is high at lower doses as shown below, but drops to less than 50% at higher doses.
Table 9.311 Bioavailability of vitamin C
| Dose (mg) | % Bioavailability |
| 200 | 112 |
| 500 | 73 |
| 1250 | 49 |
Ascorbic acid is actively absorbed by the sodium vitamin C cotransporter (SVCT). This active transport is driven by the sodium electrochemical gradient created by sodium-potassium ATPase. Ascorbic acid then diffuses into the capillary and ultimately enters general circulation. Vitamin C generally circulates as ascorbic acid.
Figure 9.311 Ascorbic acid (Asc) absorption1
Accumulation
Most water-soluble vitamins are not stored in the body. Vitamin C is not stored, but is accumulated in certain tissues in the body where it can be 5-100 times higher than found in the plasma. The table below shows the concentrations of vitamin C in different tissues and fluids.
Table 9.312 Human tissue & fluid ascorbic acid concentrations
| Organ/Tissue | Vitamin C Concentration* | Organ/Tissue | Vitamin C Concentration* |
| Pituitary Gland | 40-50 | Lungs | 7 |
| Adrenal Gland | 30-40 | Skeletal Muscle | 3-4 |
| Eye Lens | 25-31 | Testes | 3 |
| Liver | 10-16 | Thyroid | 2 |
| Brain | 13-15 | Cerebrospinal Fluid | 3.8 |
| Pancreas | 10-15 | Plasma | 0.4-1 |
| Spleen | 10-15 | Saliva | 0.1-9.1 |
| Kidneys | 5-15 |
* mg/100 g wet tissue, mg/100 mL fluids
How does the body accumulate such high levels of vitamin C? There are 2 primary mechanisms:
1. Ascorbic Acid (Ascorbate) uptake using sodium-dependent vitamin C transporter (SVCT) 1 or 2
2. Ascorbic Acid (Ascorbate) Recycling
Ascorbic Acid (Ascorbate) transport using sodium-dependent vitamin C transporter (SVCT) 1 or 2
As shown below, SVCT 1 and SVCT 2 transport ascorbic acid or ascorbate into the cell against the concentration gradient (represented by the orange wedge in the figure below). Like absorption, this uptake is driven by the action of sodium-potassium ATPase. This mechanism is saturable, meaning that at high concentrations it reaches a threshold where it cannot take up ascorbic acid any faster. Thus, there is a limit to how much can be taken up through this mechanism.
Figure 9.312 Ascorbic acid (Asc) uptake using SVCT 1 and 2 against the ascorbic acid concentration gradient2
Ascorbic Acid (Ascorbate) Recycling
In ascorbic acid recycling, ascorbic acid is oxidized to dehydroascorbic acid (DHA). DHA is then transported into the cell moving with its concentration gradient using GLUT1 or 3. Once inside the cell, DHA is reduced back to ascorbic acid, thus maintaining the DHA gradient. As a result, the cell is able to accumulate high levels of ascorbic acid. The figure below depicts ascorbic acid recycling3.
Figure 9.313 Ascorbic acid recycling
References & Links
1. Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
2. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
3. Li Y, Schellhorn H. (2007) New developments and novel therapeutic perspectives for vitamin C. J Nutr 137(10): 2171-2184.
